Power conversion devices that mutually convert power between AC and DC are applied to various applications. For example, a three-phase-two-level type is utilized as a converter that converts AC from a power system into DC, and an inverter that converts AC into DC to drive a motor. The three-phase-two-level is a technology of selecting six switches at two levels of ON and OFF, thereby performing power conversion between DC and three-phase AC.
A switching element that performs switching which is a change between ON and OFF is a semiconductor element. In general, an IGBT (Insulated Gate Bipolar Transistor) is utilized as a switching element. The three-phase-two-level inverter can be configured by six such switching elements at minimum.
The control technology of the three-phase-two-level inverter is, in general, a PWM control. The PWM control is a technology of controlling a pulse width to control a level of an output AC voltage. When, for example, an input AC voltage is Vdc, switching is performed between two values of +Vdc/2 and −Vdc/2 at a predetermined timing for each phase. Hence, the output waveform from the three-phase-two-level inverter can be a pseudo-AC waveform.
Meanwhile, a necessity for a large-scale DC power feeding of which power loss is less than an AC power feeding is increasing recently. For example, power feeding through seafloor cables, 50-Hz/60-Hz conversion, and a long-distance DC power feeding from a remote large-scale solar power generation system to a consumer location are getting attention.
According to such a DC power feeding, DC subjected to ON/OFF is an extremely high voltage like 300 kV. Conversely, the IGBTs utilized as the switching elements have a rating of substantially 6500 V. Hence, when a large number of such switching elements are connected in series to be utilized as a multi-level inverter, a voltage applied to each switching element can be decreased.